1. Field of the Invention
This invention relates generally to a powertrain for a motor vehicle vehicle, such as a hybrid electric vehicle (HEV), and, more particularly, to control of an engine shutdown and restart.
2. Description of the Prior Art
A HEV combines a conventional propulsion system having an internal combustion engine and a transmission with a rechargeable energy storage system that includes an electric motor and electric storage battery to improve fuel economy over a conventional vehicle.
Motor vehicles can be designed to employ certain aspects of hybrid electric technology to reduce fuel consumption, but without use of a hybrid powertrain. In such vehicles, called “micro-HEVs”, shutting down the engine during conditions where the engine operates at idle speed is used to reduce fuel consumption and reduce emissions in a conventional powertrain, which includes an internal combustion engine and a transmission, but no electric machine for driving the wheels. The primary condition that is checked by the micro-HEV powertrain control system before stopping the engine is that the driver has applied the brakes and the vehicle is stopped since the engine would typically be idling during these conditions in a conventional vehicle. Once the driver releases the brake pedal indicating a request for vehicle propulsion, the powertrain control system will automatically restart the engine.
Because the engine of a micro-HEV powertrain produces all the propulsion torque, it is important that the engine start-stop function be seamless and transparent to the driver. When the driver requests torque at the wheels by depressing the accelerator pedal or releasing the brake pedal, any delay in starting the engine must be minimized, yet the starter motor cannot be engaged until engine speed is at, or very near zero.
The determination of when to stop the engine is based on a variety of data such as vehicle speed, brake pedal position and clutch pedal position (in the case of a manual transmission). Once it is determined that an engine stop is necessary, the control system transitions the engine from an idle state to a ramp down state. During the ramp down state, the engine subsystem controls its actuators (spark, fuel, air, etc.) to smoothly ramp down the engine speed to zero. Furthermore, there is an engine speed above which the engine can support a change of mind (COM) engine restart. If the driver requests an engine restart during the ramp down state and the engine speed is above this “change of mind” engine speed then the engine is ramped back up to the idle speed by using the engine subsystem actuators (spark, fuel, air, etc). If the engine speed is below the “change of mind” engine speed, then the engine must be ramped down to zero (or very close to zero) and the starter motor must be used to restart the engine. This change of mind scenario can result in a long delay in restarting the engine.
The problem is, therefore, the potentially long delay in change-of-mind engine restart because the engine subsystem cannot restart without support from the starter motor. A need exists to minimize the delay in restarting the engine when a change of mind occurs with engine speed less than the “change of mind” engine speed.